1. Field of the Invention
The present invention pertains to a photoresist composition useful in photomask fabrication and in semiconductor production. In particular, the photoresist composition enables the fabrication of photomasks which exhibit a combination of dense and isolated features. The same photoresist which is used for the photomask fabrication may also be applied to semiconductor and micro electro mechanical systems (MEMS) processing.
2. Brief Description of the Background Art
Photoresist compositions are used in microlithographic processes for making miniaturized electronic components, such as in the fabrication of semiconductor device structures; for making miniaturized mechanical systems; and for making microbiological structures. The miniaturized device structure patterns are typically created by transferring a pattern from a patterned masking layer overlying the semiconductor or other substrate rather than by direct write on the substrate, because of the time economy which can be achieved by blanket processing through a patterned masking layer. With regard to the micro device processing, the patterned masking layer may be a patterned photoresist layer or may be a patterned “hard” masking layer (typically an inorganic material or a high temperature organic material) which resides on the surface of the semiconductor device structure or other substrate to be patterned. The patterned masking layer is typically created using another mask which is frequently referred to as a photomask or reticle. A reticle is typically a thin layer of a metal-containing layer (such as a chrome-containing, molybdenum-containing, or tungsten-containing material, for example) deposited on a glass or quartz plate. The reticle is patterned to contain a “hard copy” of the individual device structure pattern to be recreated on the masking layer overlying a semiconductor structure or other substrate.
A reticle may be created by a number of different techniques, depending on the method of writing the pattern on the reticle. Due to the dimensional requirements of today's semiconductor structures, the writing method is generally with a laser or e-beam. A typical process for forming a reticle may include: providing a glass or quartz plate, depositing a chrome-containing layer on the glass or quartz surface, depositing an antireflective coating (ARC) over the chrome-containing layer, applying a photoresist layer over the ARC layer, direct writing on the photoresist layer to form a desired pattern, developing the pattern in the photoresist layer, etching the pattern into the chrome layer, and removing the residual photoresist layer. When the area of the photoresist layer contacted by the writing radiation becomes easier to remove during development, the photoresist is referred to as a positive-working photoresist. When the area of the photoresist layer contacted by the writing radiation becomes more difficult to remove during development, the photoresist is referred to as a negative-working photoresist. Advanced reticle manufacturing materials frequently include combinations of layers of materials selected from chromium, chromium oxide, chromium oxynitride, molybdenum, molybdenum silicide, and molybdenum tungsten silicide, for example.
As previously mentioned, the reticle or photomask is used to transfer a pattern to an underlying photoresist, where the reticle is exposed to blanket radiation which passes through open areas of the reticle onto the surface of the photoresist. The photoresist is then developed and the patterned photoresist is used to transfer the pattern to an underlying semiconductor structure, typically using a plasma dry etching process.
As the feature size requirements for a semiconductor substrate has become smaller, and as new applications for semiconductor devices and for MEMS devices are requiring the mixing of both logic and memory features on a single chip, new issues have arisen regarding both reticle fabrication and semiconductor chip production. While the memory devices tend to have features which are closely spaced (dense), the logic devices tend to have features which are sparsely spaced (isolated). As a result, proximity effects are observed during patterning of the photoresists used for fabrication of the reticle and during patterning of the photoresists used for pattern transfer to the semiconductor substrate.
For example, in the fabrication of a reticle patterned to have feature critical dimensions (CD) in the range of about 500 nm (0.50 μm) to about 2000 nm (2.0 μm), optical proximity effects have been observed during imaging of a standard novolak-based G-line, H-line, or I-line photoresist. In particular, for a standard I-line photoresist, such as an iP3600 photoresist available from Tokyo Ohka, Tokyo, Japan, or a PF188A photoresist available from Sumitomo, Osaka, Japan, CD errors from about 20 nm to about 40 nm, which are attributable to proximity effects have been observed in the patterned photoresist which was to be used to transfer the pattern to the reticle, and were observed in the chrome of the patterned reticle.
FIG. 1 shows a typical starting structure 100 used in the fabrication of a reticle. This starting structure was generally used in the preparation of test specimens during development of the present method of fabricating reticles. Starting structure 100 is a stack of layers which includes, from top to bottom, an approximately 5,000 Å thick layer 108 of a novolak-based photoresist, iX1100P (available from Clarient Corp. of Sommerville, N.J.); an approximately 500 Å thick layer 106 of an inorganic ARC, chrome oxynitride; an approximately 200 Å thick layer 104 of a mask material which is essentially chrome; and a silicon oxide-containing substrate 102.
In view of the sizable 20 nm to 40 nm CD error which was observed in the initial fabrication of reticle test specimens, a considerable amount of effort was spent examining all of the parameters of the continuous write laser tool to determine whether these parameters might be the cause of the CD errors. The details of that work will not be discussed here, since it was determined that the laser tool parameters were not responsible for the CD errors. It was discovered that the CD errors were generated as a result of the behavior of the photoresist material during imaging and development.
FIG. 2A shows a schematic top view 200 of the photoresist layer 108 shown in FIG. 1, where a first pattern, in particular a bar pattern 202 has been written on the upper surface 201 of photoresist layer 108. The distance d1 between the bar lines 203 and 204 is about 2,000 nm (about 2.0 μm), and represents the CD which is to be controlled as tightly as possible. The thickness of each bar, 203 and 204 was about 2.0 μm. The distance d2 represents the length of the bar pattern 202 and is about 5,700 μm.
FIG. 2B shows a schematic top view 220 of the photoresist layer 108 shown in FIG. 1, where a second pattern, in particular a steps pattern 222 has been written on the upper surface 221 of photoresist layer 108. The distance d, between each half of the step pattern 222 is about 2.0 μm and represents the CD. The distance d2 is about 5,700 μm, with the length d3 of each step being about 317 μm, with the exception of the top step 224, which is about 2×317 μm. The height (thickness) d4 of the end step 226 at each end of the steps pattern 222 is about 6.5 μm, with the height d5 of the center step 224 being about 512 μm.
Since the photoresist is a positive photoresist, a cleared space is produced by exposing the photoresist to radiation and then developing the pattern created by the radiation to remove the photoresist in the irradiated area. With reference to FIG. 2A, bars 203 and 204 were irradiated by direct writing using a continuous wave laser having a half-intensity beam diameter (spotsize) of about 270 nm. With reference to FIG. 2B, each half of the steps pattern 222 was irradiated by direct writing using a the same continuous laser, where the laser was scanned over the surface 221 of photoresist 108 to produce the irradiated pattern. After writing of the pattern on the surface of starting structure, the pattern in photoresist layer 108 was developed and then transferred through underlying ARC layer 106 and chrome layer 104, to produce a chrome pattern (not shown) on the upper surface 103 of quartz substrate 102.
FIG. 3A shows the average CD for the distance d1 of the a chrome line which was produced on the upper surface of the quartz substrate 108 (in accordance with FIG. 2A), as a function of the distance traveled in direction “X” as shown in FIG. 2A. The variation in CD ranged from about 1753 nm at X=0 μm to about 1746 nm at X=2700 μm, to about 1754 nm at X=5,400 μm. The difference in CD was only about 7–8 nm over the entire length of the chrome line.
FIG. 3B shows the average CD for the distance d4 of a chrome line which was produced on the upper surface of quartz substrate 108 (in accordance with FIG. 2B), as a function of the distance traveled in direction “X” as shown in FIG. 2B. The variation in CD ranged from about 1780 nm at X=0 μm to about 1758 nm at X=2700 μm, to about 1782 nm at X=5,400 μm. The difference in CD was 23 nm over the length of the chrome line.
The difference in the CD range of the line obtained for the structures illustrated in FIGS. 2A and 2B is attributed to proximity effects which resulted from the difference in the size of the surface area of the photoresist 108 which was exposed to radiation adjacent to the line. These proximity effects are frequently referred to as photoresist loading effects.
Clearly, it would be highly desirable to be able to reduce the change in CD which is observed across a patterned reticle due to photoresist loading, as this would better enable the fabrication of a reticle where a portion of the features is dense, while another portion of the features is isolated.